We set off to investigate. First we needed to find out about
sunlight.

The light from the sun looks white, but it's made up of all the
colours of the rainbow. Sunlight travels in waves. Each colour of
the rainbow has a slightly different wave length. Some
colours travel in short waves. Other colours travel in longer
waves. Blue light waves are shorter than red light waves.

Why is the sky blue?

Earth's atmosphere isn't empty, it is
full of gases, like oxygen. When sunlight hits the Earth's
atmosphere it is scattered by all the gases in the air. Blue light
is scattered more than most other colours in the rainbow because it
has a shorter wave length. This means that we see the blue light
scattered over the sky, making the sky look blue.

Why is the sunset red?

At sunset the sky looks orange and red. The sun is lower in the
sky so the light has much further to travel to reach us. The blue
light is scattered so much that it doesn't reach us. The red light,
with a longer wavelength, does reach us, so the sky looks red.